Electroluminescent devices are opto-electronic devices where light emission is produced in response to an electrical current through the device. The physical model for EL is the radiative recombination of electrons and holes. The term light emitting diode (LED) is commonly used to describe an EL device where the current-voltage behavior is non-linear, meaning that the current through the EL device is dependent on the polarity of the voltage applied to the EL device. Both organic and inorganic materials have been used for the fabrication of LEDs. Inorganic materials such as ZnS/Sn, Ga/Bs, Ga/As have been used in semiconductor lasers, small area displays, LED lamps, etc. However, the drawbacks of inorganic materials include difficulties to process and to obtain large surface areas and efficient blue light.
Organic polymers and small organic molecules used as light-emitting materials in EL devices offer several advantages over inorganic materials, such as simpler manufacturing, low operating voltages, the possibility of producing large area and full-color displays. An efficient multilayer organic LED was first discovered by Tang et al (Tang, C. et al Appl. Phys. Lett. 1987, 51, 913-15). Conjugated polymers such as poly(phenylvinylene) (PPV) were first introduced as EL materials by Burroughes et al in 1990 (Burroughes, J. H. Nature 1990, 347, 539-41). Considerable progress has been made since then to improve the stability, efficiency, and durability of polymeric LEDs (Sheats, J. R. et al Science 1996, 273, 884-888; Cacialli, F. et al Synth. Met. 1994, 67, 157-60; Berggren, M. et al Nature 1994, 372, 444-6; Spreitzer, H. et al WO 98/27136 (1998); Holmes, A. B. et al WO 94/29883 (1994); and Heinrich B. et al, Adv. Mater. 1998, 10(16), 1340). Polymers with wide energy bandgap to emit blue light are important materials because stable, efficient blue-light-emitting materials with high brightness, are desirable for full color EL display applications. With these primary materials, it is possible to produce other colors by a downhill energy transfer process. For instance, a green or red EL emission can be obtained by doping a blue host EL material with a small amount of green or red luminescent material. The first report of blue-emission from a conjugated polymeric LED was for polydialkylfluorene (PF) (Olhmori, Y. et al Jpn. J. Appl. Phys. Part 2 1991, 20, L1941-L1943), followed by poly(p-phenylene) (PPP) (Grem, G. et al Adv. Mater. 1992, 4, 36-7). Incorporating non-conjugated spacer groups into a conjugated polymer backbone is an effective approach to break conjugation, thus increases the energy bandgap in order to emit blue light. These spacer groups usually prevent the extended conjugation and contribute to the solubility and film-forming properties of the polymer. Blue-light-emitting PPV (Aguiar, M et al Macromolecules 1995, 28, 4598-602), polythiophene (Andersson, M. R. et al Macromolecules 1995, 28, 7525-9), poly(oxadiazoles) (Pei, Q. et al Adv. Mater. 1995, 7, 559-61) and PPP (Hilberer, A et al Macromolecules 1995, 28, 4525-9) have been prepared by this approach. However, the incorporation of flexible nonconjugated spacer groups into a rigid conjugated polymer backbone reduces the stiffness of the backbone thus affecting the microscopic molecular order of the polymer (Remmers, M. et al Macromolecules 1996, 29, 7432-7445). Such groups can also act as a barrier to the injection and mobility of the charge carriers which leads to high threshold voltages and operating voltages. Thus, it is desirable to develop processable new blue-light-emitting polymers with low driving voltages for full color displays.